Comparing and Contrasting High-Entropy and Intermetallic Alloys

Not unlike high-entropy alloys (HEAs), which are having a moment now (beginning two decades ago), intermetallics had a flowering starting in the late nineteen-seventies that bloomed for three decades or so. With the benefit of hindsight, and having worked on both classes of materials, I will attempt in this presentation to compare and contrast them. Both started off targeting properties of interest for structural applications; over time, though, other physical and chemical properties piqued researchers’ interest. Similarly, like the justification offered for the investigation of HEAs, namely that there are countless possible candidates at least some of which will possess useful properties, Laves-phase compounds were once expected to be the most promising intermetallic candidates because of their sheer abundance. In retrospect, that did not quite pan out and it remains to be seen whether the abundance of HEA candidates will yield better prospects. Initially, research on intermetallics focused on ordered compounds close to their stoichiometric compositions. With time, however, it became clear that such compounds lacked the balance of properties required for high-temperature structural applications (e.g., strength, ductility/toughness, and oxidation resistance) and focus shifted first to off-stoichiometric compositions and then to two-phase microstructures consisting of ordered precipitates in a solid solution matrix. A similar evolution has occurred in the HEA field, to off-equiatomic compositions and two-phase microstructures comprising ordered intermetallic compounds in a HEA matrix. Thirty years apart, intermetallics and HEAs have in other words converged on a much earlier microstructure, namely, that of the highly optimized Ni-base superalloys. Given the current state, where are the opportunities for HEAs? I will make some speculations based on our recent research results, confining my comments to mechanical behavior.